An OFDM system can be employed in high rate wireless telecommunication systems. In a transmitter of the OFDM system, a data stream having a high transmission rate is divided into various data streams, each having a relatively low transmission rate. The data streams are simultaneously transmitted in parallel using a plurality of orthogonal sub-carriers. The data streams are then recombined to obtain the original data stream in a receiver of the OFDM system.
FIG. 1 illustrates a discrete-time baseband equivalent model of a typical OFDM system 100, which includes a transmitter 110, a receiver 120, and a channel 130. The transmitter 110 includes an encoder 111, a serial-to parallel (S/P) converter 112, an inverse fast Fourier transform (IFFT) module 113, a parallel-to-serial (P/S) converter 114, and a digital-to-analog (D/A) converter 115. The receiver 120 includes a decoder 121, a P/S converter 122, a fast Fourier transform (FFT) module 123, an S/P converter 124, and an A/D converter 125.
In the transmitter 110, the encoder 111 first encodes an input bit stream into a symbol stream, using, for example, M-QAM (Multiple-level Quadrature Amplitude Modulation) modulation. The symbol stream is then converted into N parallel symbol streams in the S/P converter 112 (N is a number of points of IFFT as well as a total number of sub-carriers to be used in the OFDM system), wherein the kth symbol stream is to be transmitted on the kth sub-carrier. Then the N parallel symbol streams are sent to the IFFT module 113 for IFFT modulation. The output of the IFFT module 113 is serialized at the P/S converter 114. The output of the P/S converter 114 is converted into analog signals at the D/A converter 115 before the signals are sent to the receiver 120.
In the receiver 120, the received analog signals are first converted to digital signals at the A/D converter 125. Then the digital signals are converted to parallel signals at the S/P converter 124 and supplied to the FFT module 123 for demodulation. Symbol streams obtained by demodulation are serialized at the P/S converter 122 to obtain a symbol stream corresponding to the symbol stream output from the encoder 111. Then the symbol stream output from the P/S converter 122 is decoded at the decoder 121 to obtain an output bit stream corresponding to the input bit stream.
In such an OFDM system, any imperfection at the transmitter and receiver will cause performance degradation because such imperfection will destroy the orthogonality of the sub-carrier waveforms. Without the orthogonality, when the arriving signals are carried down to the baseband at the receiver, Inter-Carrier Interference (ICI) occurs. ICI refers to signal interference from one sub-carrier to others, mostly to neighboring sub-carriers.
In an OFDM transmitter, many factors such as, oscillator inaccuracies, noise, clock jitter etc. cause the imperfection. In such situations, the orthogonality of the sub-carriers at the transmitter no longer exists. The transmitter is also sensitive to RF section design choices, circuit board layout and implementation, and component variations and changes. Any imperfection or inaccuracies in those aspects will cause the orthogonality to be lost at the transmitter. The situation is further complicated by the different modulation types and frequency bands that the OFDM system may employ. Since ICI is one of the critical factors that affect the performance of the OFDM system, measurement of ICI becomes an important aspect for assessing the performance of the OFDM system.